1. Field of Invention
The present invention relates in general to a control circuit of power converter, and more particularly, to synchronous rectifying control circuit for power converters.
2. Description of Related Art
FIG. 1 shows a circuit schematic of an offline resonant power converter. It includes a power transformer 10 to provide isolation from AC line input to the output of the power converter for safety. The soft switching of the resonant power converter achieves high efficiency and low EMI (electric-magnetic interference) performance. Transistors 20 and 30 develop a half bridge circuit to switch a resonant tank. The resonant tank is formed by the power transformer 10, an inductor 15 and a capacitor 40. The inductor 15 acts as a primary-side leakage inductance of the power transformer 10 and/or an inductor device. The inductance L of the inductor 15 and the capacitance C of the capacitor 40 determine the resonance frequency f0.
                              f          0                =                  1                      2            ⁢            π            ⁢                                          L                ×                C                                                                        (        1        )            
The power transformer 10 transfers the energy from the primary side to the secondary side of the power transformer 10. Rectifiers 41 and 42 rectify the switching voltage of the power transformer 10 into a capacitor 65. A DC voltage VO is thus outputted at the output terminal of the power converter. The output load of the power converter determines the Q value of the resonant tank through the power transformer 10. FIG. 2 shows the normalized amplitude of the output power as a function of the switching frequency. The maximum power is delivered to the output terminal of the power converter when the switching frequency is operated at the resonant frequency f0. If the switching frequency is lower than the resonant frequency f0, then the resonant tank will become capacitive impedance. The resonant tank will become inductive impedance when the switching frequency is operated higher than the resonant frequency f0. The voltage-controlled oscillator (VCO) of the power converter will control the switching frequency in between the resonant frequency f0 and the maximum frequency fM to ensure the feedback circuit of the power converter is operated under a linear system. Besides, controlling the switching frequency below the maximum frequency fM is to make sure a soft switching mechanism. The detail skill of the resonant power converter can be found in text book “Resonant Power Converters” by Marian K. Kazimierczuk and Dariusz Czarkowski, 1995 by John Wiley & Sons, Inc.
In recent development, applying the synchronous rectifier on the secondary side of the power transformer is a further approach to achieve a higher efficiency power conversion, such as “Control circuit associated with saturable inductor operated as synchronous rectifier forward power converter” by Yang, U.S. Pat. No. 7,173,835. However, the disadvantage of this prior art is an additional power consumptions caused by saturable inductors, etc. The object of present invention is to provide a synchronous rectifying circuit for resonant power converters to achieve higher efficiency.